1. Field of the Invention
This invention relates to the purification of gas streams by removal therefrom of acidic gases, such as nitrogen oxides, carbon dioxide, hydrogen sulfide, sulfur dioxide, halogen gases, halogen acid gases, organic acid gases such as formic acid, acetic acid and the like, by means of vapor phase reaction between the acidic gases and an amine, followed by separation of the reaction products from the gas streams. The process and apparatus of the invention have particular utility in removal of oxides of nitrogen from chemical process tail gases, combustion products of gas, coal and oil fired power generators and the like, exhaust gases of metal pickling operations, and from other gaseous emissions wherein the problem of nitrogen oxide pollution is present.
2. Description of the Prior Art
Increasingly stringent requirements for removal of pollutants from gases discharged to atmosphere have made obsolete many conventional systems and processes for purification of exhaust gases from industrial process which were effective in removing up to about 98% of the pollutants. In the removal of acidic gases it has long been customary to contact a gas stream with alkaline scrubbing solutions containing ammonia, amines, urea, alkali metal hydroxides, and/or alkaline salts. Representative patents disclosing processes of this type include U.S. Pat. Nos. 2,608,462; 2,613,132; 2,638,405; 2,889,197; 2,955,909; 3,161,461; 3,329,478; 3,463,603; 3,502,428; 3,635,657; 3,653,809 and 3,653,810.
Removal of acidic gases from a gas stream by passage through a zeolite molecular sieve or a fluidized bed of an absorptive solid impregnated with a high boiling point amine is disclosed in U.S. Pat. Nos. 2,818,323 and 3,508,382.
Catalytic conversion of nitrogen oxide gases is disclosed in U.S. Pat. Nos. 3,425,893 and 3,567,367.
The efficient removal (i.e. at least 99 percent efficiency) of nitrogen oxides has proven to be more difficult than removal of other acidic gases such as carbon dioxide, hydrogen sulfide, and sulfur dioxide. Considerable attention has recently been directed to this problem, and reference may be made to the following articles in the Journal Of The Air Pollution Control Association:
Volume 19, No. 10, October 1969, pages 791-794 PA1 Volume 20, No. 5, May 1970, pages 303-306 PA1 Volume 21, No. 3, March 1971, pages 122-127 PA1 Volume 22, No. 6, June 1972, pages 471-472 PA1 Volume 74, No. 4, Jan. 25, 1971, 15556n PA1 Volume 74, No. 24, June 14, 1971, 140021w PA1 Volume 76, No. 14, Apr. 3, 1972, 76090u PA1 Volume 77, No. 22, Nov. 27, 1972, 143502r PA1 Volume 78, No. 4, Jan. 29, 1973, 19849r PA1 Volume 78, No. 6, Feb. 12, 1973, 33476y PA1 Volume 78, No. 10, Mar. 12, 1973, 61806z PA1 Volume 78, No. 18, May 7, 1973
Recent Chemical Abstracts references relating to control of nitrogen oxide emissions include the following:
The above-mentioned article in Journal Of The Air Pollution Control Association, Vol. 21, No. 3, March 1971, pages 122-127, discusses the rate of conversion of NO to NO.sub.2, points out that the rate is much slower at low concentrations, that the solubility rate of NO.sub.2 in water or alkaline solutions is much slower at low concentrations, and concludes that reduction of stack gas concentrations of nitrogen oxides below about 200 ppm in liquid absorption equipment of practical dimensions is impractical.
In stationary power generating equipment utilizing fossil fuel it appears from the above literature sources that emphasis has shifted to changes in design and combustion techniques rather than reliance on absorption equipment, in order to reduce nitrogen oxide emissions.
The use of ammonia in accordance with prior art disclosures, either in aqueous solution or in the presence of a solid catalyst, for the reduction of oxides of nitrogen, inherently involves some reaction in the vapor phase because of the volatility of ammonia gas. However, the exothermic reaction of ammonia with oxides of nitrogen is of an entirely different type than that of amines with nitrogen oxides. According to Air Pollution Control -- Part I, Wiley Interscience Series of Texts and Monographs, (1971) pages 80-88, catalytic reduction of nitrogen oxides by ammonia produces a gas stream vented to atmosphere containing 80 ppm of oxides of nitrogen and 10 ppm of ammonia. It is therefore apparent that ammonia cannot achieve substantially complete removal of nitrogen oxides in a reactor of practical size.
At the present time the consensus of workers skilled in the art is that known methods of removing nitrogen oxides from gas streams by absorption in solutions or by solids are not effective in meeting recent requirements in air pollution control. Catalytic reduction of nitrogen oxides is considered to be commercially feasible, but it is well known that such catalysts are expensive, subject to poisoning by other pollutants which may be present in the gas stream being cleaned, and require precise control and maintenance.
It is therefore apparent that there is a definite need for a simple, efficient and reliable method and apparatus for removing acidic gases from gas streams before discharge thereof to atmosphere.